The present invention relates to surface modification of metal powder, such as that used as fuel in pyrotechnic compositions, in order to improve the stability of such compositions during storage and so reduce hydrogen out-gassing. It applies principally to magnesium (Mg) powder fuels as these are the least stable, but may also be applied to Aluminum (Al) powder fuel. The invention provides also a metal powder having an improved stability.
Metal-fuelled pyrotechnics are used to produce light, sound, luminosity or infra-red emissions. Magnesium is a preferred fuel due to its high reactivity, low boiling point and high heat of combustion.
The magnesium powder fuel may be made by twin-fluid atomization using compressed inert gas to atomize liquid magnesium to produce spherical particles; or by mechanical methods, such as rasping or milling solid ingot to produce irregular or flaked particles. The particle size range normally used is typically 75 microns or less and may have a surface area of about 3000 to 4000 cm2 per cm3.
The magnesium oxide coating, which is present on the particle surfaces, is porous and non-protective. The very desirable high reactivity of magnesium powder is to a great degree due to the high surface area of the particles and the non-protective nature of the oxide coating. However, the high reactivity of magnesium powder makes the pyrotechnic compositions containing it very vulnerable to degradation by moisture introduced during and after manufacture. The result is that highly flammable hydrogen gas may be released during the storage period before deployment, typically by the following reaction:Mg+2H2O=>Mg(OH)2+H2 This out-gassing can cause swelling and de-lamination of the compacted pyrotechnic composition and in extreme cases can damage the hermetic sealing arrangements, allowing more moisture ingress and further shortening the useful storage life of the pyrotechnic. Most importantly, the presence of hydrogen gas in the vicinity of the pyrotechnics is very dangerous as it can very easily ignite and cause premature ignition of the pyrotechnics and possible injury to personnel in the vicinity
Known methods for reducing out-gassing in pyrotechnics usually involve the application of organic coatings to the metal powder fuel. A typical example is disclosed in U.S. Pat. No. 6,174,391, which comprises coating magnesium powder with at least one ethylene and vinyl acetate co-polymer. This, and similar organic coating methods, aim to form a physical moisture barrier on the magnesium particles and thus reduce the hydrogen out-gassing reaction by preventing contact between the coated magnesium powder and any moisture present. Such methods have the disadvantage that at least one additional manufacturing step is introduced, which increases cost and complexity; particularly as the additional steps usually use liquid solvents which need to be subsequently removed. Also, the effectiveness of organic coatings reduces with time as moisture gradually penetrates the coating and exposes the underlying active magnesium substrate to attack.